The effects of thyroxine (T4) on several molecular properties of thyroxidine-binding globulin (TBG) have been evaluated. Changes in sedimentation constant and relaxation time indicate that TBG becomes more compact when T4 is bound and this is associated with increased resistance to denaturation by acid or guanidinium chloride. T4 binding also alters the emission and excitation spectra of TBG, reflecting differing environments of its 4 tryptophanyl residues. These studies provide a model for allosteric effects of thyroid hormone binding on receptor proteins.